The present invention relates to improvements in the measurement and indication of influences of gases within a sealed cavity such as one which is evacuated and is intended to sustain a relatively hard vacuum, and, in one particular aspect, to unique and improved vacuum-monitoring processes and equipment involving a thin-film deposit of a getter material, such as barium, insulatedly supported within an evacuated cavity and exhibiting its electrical resistance to external detection and signalling circuitry which responds thereto and to its changes induced by reactions with unwanted gas, whereby contamination and depletion of desired vacuum conditions are monitored indirectly.
Uses of so-called "getters" has long been commonplace in connection with incandescent lamps and electronic vacuum tubes, wherein high-vacuum and contamination problems have been alleviated somewhat by their removal of residual gas and, in the case of so-called "keepers", by their further combination with gas subsequently liberated. Among the materials which combine readily with gases, and have desirable gettering properties, are barium, magnesium, zirconium, red phosphorous, aluminum and cerium, and tantalum and zirconium have outstanding capacity of absorb gases when raised to high temperatures. It has also been known to hold a low pressure in other enclosures, such as the reference-pressure cavity of a differential-pressure device, by means of a getter pump which tends to effect the error-inducing consequences of outgassing or leakage into that cavity. For such purposes, a clean surface of highly reactive getter material, such as barium, is vapor-deposited with the aid of resistance or induction heating within the cavity before or after it is sealed off. Thereafter, however, the extent to which the reference cavity may have lost its useful vacuum, despite initial presence of a deposited getter layer, cannot be determined by the user, and the pressure sensing may in fact involve significant error.
In accordance with certain aspects of the present teachings, the resistance changes in a deposited layer of barium getter or the like are sensed externally of an evacuated cavity, and implementation of that practice calls for the barium layer to be in good electrical contact with sealed feedthrough terminals and to be essentially insulated from other parts whose resistance might tend to by-pass and render uncertain the sensing of resistance of getter layer material between those terminals. Control of the deposit of the getter layer is thus quite important, and, to that end, a preferred deposition practice akin to that used with metal-envelope vacuum tubes is adapted to a localized and directionalized application of the layer to critical surface areas; specifically, a suitably-coated metal ribbon, such as one of barium-coated tantalum or the like, with mechanical shielding disposed nearby, is heated by an electrical current and resulting barium vapor from its coating is directed to those surface areas.